1. Field of the Invention
The present invention relates to a technique of shortening a recovery time of information on a route after maintenance work or in the event of a temporary failure in a network comprising a plurality of routers.
2. Description of the Related Art
In this specification, a network such as the Internet is a router network comprising a plurality of routers. Each router relays a transmitted frame to an adjacent router in accordance with information on a route in order to pass on the frame to a destination terminal. The frame is originated from a terminal such as a personal computer and transmitted to the destination terminal.
FIG. 17 is a block diagram showing the configuration of the conventional router. A frame-receiving unit 6 outputs route information received from a transmission line by way of an IF unit 2 to a route-information management unit 8. Route information is information on a route including a destination and a distance. Information on a route is transmitted by an adjacent router in accordance with a predetermined protocol such as RIP or OPSF. The route-information management unit 8 stores the information on a route in a route-information database 10. The route-information management unit 8 also passes on the information on a route to an adjacent router by way of the frame-transmitting unit 15 and an IF unit 4. In addition, the route-information management unit 8 determines information on relay routes and stores this information in a relay-route-information database 12. The information on relay routes shows relay routers for destinations of the route information stored in the route-information database 10.
On the other hand, a frame-receiving unit 6 outputs a frame received from the transmission line through the IF unit 2 to a frame-relay-processing unit 14. The frame-relay-processing unit 14 searches the relay-route-information database 12 for a transmission line to the destination of the received frame and passes on the frame to the transmission line relay-route-information database 12.
When detecting an abnormality such as loss of route information supposed to come from an adjacent router or when again receiving information on a route upon recovery of information on a route, the route-information management unit 8 informs an adjacent router of such a change in information on a route.
FIG. 18 is a diagram showing how a change in information on a route flows in the event of a temporary halt. When the operation of a router 16#1 is temporarily halted due to version-upgrading work, for example, a router 16#2 adjacent to the temporarily halted router 16#1 no longer receives information on a route from the temporarily halted router 16#1 on a temporary basis. When detecting the temporarily halted operation of the router 16#1, the adjacent router 16#2 informs further adjacent routers 16#3 and 16#4 of a change in information on a route. When receiving the change in information on a route from the adjacent router 16#2, the further adjacent routers 16#3 and 16#4 report the change in information on a route to still further adjacent routers 16#6 and 16#5 respectively. In this way, the change in information on a route is disseminated throughout the entire network.
In accordance with Japanese Patent Laid-open No. Hei 10-32234, by making OPSF packet headers uniform, information on a route can be exchanged at a high speed. In accordance with Japanese Patent Laid-open No. Hei 6-334735, dissemination of a temporary unstable state of information on a route is suppressed by an apparatus alone by periodically monitoring a change in information on a route and not reporting such a change in the event of such a temporary unstable state such as an intermittent failure. In accordance with Japanese Patent Laid-open No. 2000-307605, a routing table used for transferring an IP packet in the event of a failure is provided separately from a routing table used for transferring an ordinary IP packet in order to increase the speed of a failure recovery.
However, the conventional router raises the following problems. The operation of a router is temporarily halted during maintenance of the router or in the event of a temporary failure occurring in the router. Even for a case in which the original configuration can be restored immediately, a change in information on a route is disseminated throughout the entire network in spite of the fact that routers in the network do not need to know such a change. In this case, the larger the number of routers included in the network, the greater the effect of the problem, or the longer the time it takes to stabilize information on a route. In addition, in accordance with Japanese Patent Laid-open No. Hei 10-32234, information on a route can be exchanged at a high speed. However, there is raised a problem of a long time to stabilize information on a route. Furthermore, in accordance with Japanese Patent Laid-open No. Hei 6-334735, since an apparatus needs to periodically monitor a change in information on a route condition by itself, it is necessary to set the monitoring period in advance at a value including a time required to recover a router from an intermittent failure. Thus, if a timer is set at a short period in spite of the fact that the maintenance time and/or the time required to recover a temporary failure varies, a change in information on a route may be reported immediately even though there is raised a problem that an intermittent failure cannot be handled completely. If the timer is set at a long period, on the contrary, detection of a real failure may be too late, causing another problem that dissemination of a change in information on a route throughout the entire network is also too late. Moreover, a technology disclosed in Japanese Patent Laid-open No. 2000-59426 strives for continuation of communication instead of shortening a time required to stabilize information on a route. Thus, there remains the problem described above that a change in information on a route is disseminated throughout the entire network in spite of the fact that routers in the network do not need to know such a change.